Oxygen is dissolved in solution. To take an example of blood, blood contains oxygen bound to hemoglobin, as well as oxygen as dissolved oxygen. For example, detection and analysis of a target substance in blood (particularly in serum or plasma) employs a measurement system using an oxidative enzyme. The measurement system utilizes oxygen in a reaction solution to allow enzyme reaction to proceed. In this case, in a tightly closed space not in contact with air, oxygen that can be utilized for the reaction is only oxygen already present in the specimen and the reaction solution, and there seems to be almost no additional supply of oxygen.
However, in such a state without any oxygen supply, problem can occur when a reagent for analyzing the target substance is a dry reagent, reaction space is the tightly closed space, and then, specimen is flowed in without being diluted. Specifically, it is very unlikely that an intended concentration range of the target substance can be obtained by using only the dissolved oxygen. For example, the amount of dissolved oxygen in blood is approximately from 0.6 to 0.7 mmol/L. However, when considering an example of uric acid or creatinine as a target substance to be measured in a blood specimen, the concentration of uric acid or creatinine, respectively, is approximately 10 mg/gL or 7 mg/dL. That is, since a generally required measurement range for both uric acid and creatinine is 0 to 20 mg/dL, the measurement system as described above cannot satisfy specifications required by the market. Thus, in such a case, instead of using the specimen in its undiluted state, the specimen may be diluted up to an oxygen concentration capable to utilize an oxidative enzyme to supplement oxygen supply before reaction for analysis of a target substance.
Meanwhile, Examined Japanese Patent Application Publication No. H4-76679 has disclosed an analytical instrument in which oxidative enzyme reaction is similarly utilized not in a tightly closed space but in an open system. The analytical instrument uses a dry reagent layer and has an oxygen supply layer between a support and the reagent layer. In addition, the oxygen supply layer has a porous hydrophobic structure because of suppression of uneven color development. The Patent Literature has stated that the porous hydrophobic structure is made of nonwoven fabric, hydrophobic woven fabric or paper, metal and nylon mesh, membrane filter, hydrophobically surface-treated glass filter, ceramic, or the like.